Not Applicable
Not Applicable
Not Applicable
This invention relates to a contacting device used as packing in mass and heat exchange equipment like distillation column or used as a static mixer to blend fluids. Any commercial chemical process requires mass and heat transfer between fluids. These processing methods can include distillation, absorption, adsorption, desorption, stripping, drying, gas cleaning, humidification, dehumidification and direct contact cooling or heating.
The equipment, which carries out such operation, is required to provide a contacting medium or device for generating effective transfer rates. These contacting media or devices must be capable of providing sufficient interfacial area over which two fluids can interface with one another. Further the area should be configured in a relatively small volume. The higher density of effectively available area provided by a contacting device reduces the size of the equipment and consequently reduce manufacturing cost. In addition, the contacting device should be flexible in its configuration so as to be useful for multitudes of different fluids handled in chemical industry. These contacting devices are generally hollow or solid bodies of predetermined size, shape, configuration and orientation. The xe2x80x9cstructuredxe2x80x9d packing has individual members oriented in a predetermined fashion The xe2x80x9crandomxe2x80x9d packing is dumped and has no specific orientation within the equipment.
An ideal contacting device should have three major strengths:
It should provide a wetting film of liquid covering all of provided area.
It should provide least resistance to flow of rising gas.
Each of the contacting phases should remain well mixed at any cross-section of packing providing uniform profile of flow, concentration and temperature.
To some extent, these are contradictory requirements. Provision of higher area for liquid film requires dense structure. This reduces voidage available for gas flow and increases gas pressure drop. Liquids have natural tendency to consolidate. Any aids like structural protrusions to split and spread liquids, become obstacles in the gas path increasing its pressure drop. Prior art describes numerous packing structures based on corrugated sheets layout. These are at best workable compromisesxe2x80x94far from perfection. Traditionally known prior art contacting device like structured packing is built with layers of zigzag folded metal sheets (corrugated or pleated sheets) as described in WO 90/10497 and U.S. Pat. No. 4,926,050 (Meier). These sheets are arranged in layers parallel to the direction of the axis of flow but the folders are inclined to the axis. The layers are arranged alternately making multiple channels crossing each other.
The prior art corrugated sheet layout suffers from few deficiencies, which reduce and limit the performance in terms of contacting efficiency. The deficient performance is mostly due to inherent geometry of corrugated sheet layout. Liquids have a natural tendency (due to surface tension) to merge together into a consolidated stream. This xe2x80x9csheetingxe2x80x9d of liquid film, leads to under-irrigated portions on the corrugated sheet thus wasting some of the available sheet area. The morphology (inherent spatial geometry of construction) of prior art packing does not particularly avoid this xe2x80x9csheetingxe2x80x9d of liquid and consequent lowering of contact efficiency.
Another notable deficiency of corrugated sheets is, limited intermixing of cascading streams at any location. Even the liquid on one side of the corrugated sheet cannot mix with the liquid on the opposite side.
Many inventors have addressed these problems in the past. In one embodiment, the corrugated packing sheets have number of holes drilled for the liquid to cross-over (U.S. Pat. No. 5,876,638, Sunder). In another scheme, xe2x80x9cwxe2x80x9d and xe2x80x9cvxe2x80x9d shaped shutter openings are formed on the corrugated sheet to create liquid drip points and improve mixing.(U.S. Pat. No. 4,676,934, Seah). Yet another scheme suggests provision of oblique deflection surfaces that are projected from corrugations to mix liquid flowing on the opposite side of the same sheet (U.S. Pat. No. 5,063,000, Mix and U.S. Pat. No. 5,407,607, Mix).
However, all such holes, shutters or deflections reduce the available sheet area for the liquid film. Further the sudden discontinuity created by these features in the path of the liquid film distorts the already created film. Thus all these improvements become counter-productive by disturbing the very liquid film that one had aimed to stabilize. Further the projections suggested in these improvements add to pressure drop for the rising vapor or gas.
The liquid flowing in one set of channels of the corrugated sheets tends to flow down in the same location without much of a lateral movement. The low level of lateral mixing of the fluids (in the direction perpendicular to the equipment axis) also reduces contact efficiency due to non-uniform concentration gradient.
The low lateral movement of the liquid on the corrugated sheet layout tends to maintain uneven flow profile. The uniformity of liquid spread and in turn exchange efficiency of packing depends totally on initial distribution. This has to be provided by additional devices like liquid distributors located above the corrugated packing. This essentially limits the range of operation of packing, as the distributor normally looses performance much before the efficiency limit of packing is reached. Thus corrugated sheet packing becomes as good as the distributor above it.
The inclined channels at the ends of the bundle of the prior art packing tend to put the liquid towards walls of the equipment. The morphology of the arrangement of corrugated sheets does not provide a way to transport this wall liquid back into the bulk of packing. The severity of this problem was long recognized. One scheme by Billingham (U.S. Pat. No. 5,700,403) suggests provision of recessed vertical edges in the corrugation to divert wall liquid. In another scheme, the folded layers are coupled by redirector elements for the edge seeking liquid to reduce the migration of liquid towards the equipment wall.(U.S. Pat. No. 5,441,793, Suess).
Both the schemes are complicated to fabricate and add few extra steps in manufacture thereby increasing the cost.
Apart from liquid distribution, the uniform distribution of rising vapor or gas and its lateral mixing is also a concern in the corrugated sheet layout. The vapor tends to flow in channels without mixing with the vapor in the adjacent channels in a corrugated sheet bundle.
This problem is addressed in one embodiment by providing fan like vane elements to aid transverse (lateral) mixing (U.S. Pat. No. 5,158,712, Wilhem). In another embodiment, the elementary triangular area in the folding has two cut edges that are deflected in a flap like manner to form vortex packing (U.S. Pat. No. 5,500,160, Suess). However these embodiments still retain legacy of corrugated or pleated sheets morphology. The embodiments suggest deflection of partly cut portions in the corrugated channels to form vanes. These embodiments still achieve limited split and mixing of vapor phase and are undue complex and hence costly. In another scheme, angled pegs are projected at various places from one sheet to adjoining sheet forming a bridge for lateral movement of both liquid and vapor (U.S. Pat. No. 5,975,503, Chuang). All these methods add to cost and complexity of the packing.
In order to spread the liquid and the vapor uniformly across the radial plane, the corrugated sheet bundles are required to be kept short. Further, the prior art bundles have to be stacked so that the channels are rotated by 90 degrees to each other. This feature essential for distribution of liquid is at the cost of additional pressure drop for the rising vapor. The vapor has to abruptly change its direction at each of the numerous junctions between the short bundles. This high turbulence of vapor at the junctions is one of the reasons of liquid carryover, back mixing of the liquid, foaming and is a probable place where column flooding starts.
The traditional packing is made so as to fit into a specific size or diameter of equipment. The corrugated sheets are cut and assembled to fill the entire cross section of the column. The individual sheets"" width is fixed by the diameter of the equipment and the location of that particular sheet in the bundle. If the capacity or processing requirements change, the equipment diameter changes. The originally purchased packing cannot be used for a new diameter. This diameter specific nature of the prior art packing limits its use in any other equipment of different size and shape. This reduces flexibility or reusability of the prior art packing for varying requirements.
Most of the deficiencies enumerated above arise due to the inherent morphology of the corrugated sheet layout. The known improvements and variants of the corrugated packing are end-of-the-pipe solutions. They aim at solving problems by going around it than eliminating it.
Therefore, there is a need to develop a contacting device or structured packing that mitigates the drawbacks of the prior art packing. The new way aims to break off from the prior art corrugated sheets layout by providing for a structured packing comprising of a haystack like wound bundle made from discrete, contoured, and preshaped elements.
The present invention is based on a totally different morphology compared to the known corrugated sheet packing morphology. The invention in its generalized form, provides a structured packing comprising of a haystack like wound bundle of discrete elements. The subdivided surface of the individual element is given a spatial orientation (contour) to achieve substantial wetting by the liquid. The contoured surfaces are positioned as to give an overall helical shape to the element. The oppositely inclined elements are arranged in two (or more) layers one above the other and then wound into a haystack like bundle. This uniform spatial arrangement distributes liquid evenly into many trickle streams flowing down the matrix of many inclined and interlinked elements. The gas rises through multidirectional twisting spaces created by voids in the matrix of the bundle. The preferred embodiment uses strips as the element for constructing the contiguous bundle. The present invention will be useful as a phase contacting media for distillation, absorption, stripping, liquid-liquid extraction, multiphase reactors (packed or trickle towers). It can be also used for in-pipe static mixing or for mixing fluids in a vessel. The morphology of this invention is so versatile, that it can be used as a liquid distributor or a mist eliminator.
Further objective of the present invention is to make more efficient packing that maximizes the liquid spread on the provided structured packing area. In the preferred embodiment, the strip surface is so contoured and roughened as to guide the small liquid stream trickling down along both sides of the strip, to spread and wet maximum of its surface area. The liquid flow splits as tiny streamlets on the multitudes of strips quickly attaining isotropic (uniform in all directions) flow. The liquid consolidation or xe2x80x9csheetingxe2x80x9d observed in prior art corrugated sheet packing is thus effectively tackled in this invention.
According to another objective of the invention, the streams of rising gas smoothly and repeatedly split and recombine in the gently twisting spaces in the voids of the matrix of the inclined strip. The gas is divided into numerous directions. This flow path keeps the gas well mixed giving uniform concentration gradient. The present invention avoids sudden and abrupt change of direction of the gas, both within and between the stacked bundles. Further, absence of any protruding obstacles in the gas path helps in reducing pressure drop of the rising gas. This feature has a marked advantage over the prior art wherein the gas abruptly changes direction at the junctions of the bundles.
Another object of this invention is to allow lateral movement of the liquid to achieve uniformity in terms of composition at any cross section in the packed section. This objective is achieved due to an inherent geometry, in which the inclined strips in one layer are in contact with many strips in neighborhood layers. This morphology distributes the liquid quickly and uniformly across the entire cross section of the packing and to an extent self corrects any maldistribution. This unhindered lateral movement of the liquid and continuous self-correction of maldistribution is a distinct advantage. In contrast, the prior art packing is totally dependent on extra devices such as the liquid distributors.
Additional object of this invention is to reduce excessive migration of trickling liquid towards the walls of the equipment. The inherent morphology of this invention, helps to transfer the wall liquid back into the bulk packing. The adjacent strips in the bundle are touching each other in several directions and at multiple points. This helps in transferring the excess liquid from one strip to all adjacent strips quickly. This generates a net movement of the liquid from flooded area to under-irrigated area. The strips in the outer most layer in the bundle, which are touching the equipment wall or wall wipers collect the liquid. The lateral liquid migration from strip to strip then provides a quick way for the wall liquid to travel into the bulk packing. In the corrugated sheet, the foldings at the outer edges deliver liquid on the equipment wall. This liquid tends to travel along the wall bypassing the bulk packing. The prior art requires complex redirectors to redirect wall liquid.
Further, the packing morphology makes it possible to add or remove elements (layers of strips) to suit changes in the equipment diameter. The present invention has morphology that makes the packing reusable and flexible for such size variations. This is a distinct advantage over the prior art corrugated sheets that are essentially diameter-specific. Further object of this invention is to make contacting device versatile. In the present invention elements can be in variety of shapes like strips, rods, flattened tubes, and rectangular plates. By manipulating the geometric parameters, type of element and thus the morphology of the bundle, one can make various devices like structured packing, random packing, liquid distributor, mist eliminator, static mixer, and agitator. Such a multiple and versatile application range is not possible with the prior art devices.